This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this Background of the Invention section are to be read in this light and are not to be understood as admissions about what is, or what is not, prior art.
Many potential applications exist for a non-invasive technique to monitor respiration, heartbeat or both. Doppler radar, operating at microwave frequencies in the range of 1-10 GHz, has long been suggested as a means to accomplish this (see, e.g., Lin, “Noninvasive Microwave Measurement of Respiration,” Proc. IEEE, vol. 63, p. 1530, October 1975; Pedersen, et al., “An Investigation of the employ of Microwave Radiation for Pulmonary Diagnostics,” IEEE Trans. Biomed. Eng, vol. BME-23, pp. 410-412, September 1976; Griffin, “MW Interferometers for Biological Studies,” Microw. J., vol. 21, pp. 69-72, May 1978; Lin, et al., “Microwave Apexcardiography,” IEEE Trans. Microw. Theory Tech., vol. MTT-27, pp. 618-620, June 1979; and Chen, et al., “An X-band Microwave Life-Detections System,” IEEE Trans. Biomed. Eng., vol. BME-33, pp. 697-701, July 1986).
More recently, radio frequency (RF) technology developed for mobile telephones (i.e., cellphones) has been applied to implement such devices (see, e.g., Droitcour, et al., “A Microwave Radio for Doppler Radar Sensing of Vital Signs,” in IEEE MTT-S Int. Microwave Symp. Dig., 2001, vol. 1, pp. 175-178; Lohman, et al., “A Digital Signal Processor for Doppler Radar Sensing of Vital Signs,” in Proc. IEEE 23rd Annual Engineering in Medicine and Biology Soc. Conf., 2001, vol. 4, pp. 3359-3362; Boric-Lubecke, et al., “10 GHz Doppler Sensing of Respiration and Heart Movement,” in Proc. IEEE 28th Annual Northeast Bioengineering Conf., 2002, pp. 55-56; Droitcour, et al., “Range Correlation Effect on ISM Band I/Q CMOS Radar for Non-contact Vital Signs Sensing,” in IEEE MTT-S Int. Microwave Symp. Dig., 2003, vol. 3, pp. 1945-1948; and Droitcour, et al., “Range correlation and I/Q performance benefits in Single-Chip Silicon Doppler Radars for Noncontact Cardiopulmonary Monitoring,” IEEE Trans. Microw. Theory Tech., vol. 52, pp. 838-848, March 2004).
Mobile telephone RF technology has also been generalized to sensing of multiple subjects (see, e.g., Boric-Lubecke, et al., “Doppler Radar Sensing of Multiple Subjects in Single and Multiple Antenna Systems,” in Proc. 7th Int. Conf. Telecommunications in Modern Satellite, Cable, Broadcasting Services, 2005, vol. 1, pp. 7-11; Smardzija, et al., “Applications of MIMO Techniques to Sensing of Cardiopulmonary Activity,” in Proc. IEEE/ACES Int. Conf. Wireless Communications, Applied Computational Electromagnetics, 2005, pp. 618-621; and Zhou, et al., “Detection of Multiple Heartbeats Using Doppler Radar,” in Proc. IEEE Int. Conf. Acoustics, Speech, Signal Processing (ICASSP), 2006, vol. 2, pp. II-1160-11-1163). A particularly comprehensive discourse on the subject including physiological background can be found in Droitcour, Non-Contact Measurement of Heart and Respiration Rates with a Single-Chip Microwave Doppler Radar, Ph.D. Dissertation, Stanford University, 2006.
Most of the studies to date have been done in the form of laboratory experiments under ideal conditions, so substantial concern exists that the technology can ever be developed into reliable products. Some potential problems that have yet to be addressed include the effects of background scatter, the motion of the subject as well as the background and interference between the respiration and heartbeat signals. Background scatter both from the ambient surroundings as well as from parts of the subject's body exclusive of the relevant chest-wall area (see, e.g., Ramachandran, et al., “Reconstruction of Out-Of-Plane Cardiac Displacement Patterns as Observed on the Chest Wall During Various Phases of ECG by Capacitance Transducer,” IEEE Trans. Biomed. Eng., vol. BME-38, pp. 383-385, April 1991) adds a component to the desired signal that must be dealt with. Gross motion of the subject, as well as other objects in the background will introduce undesired dynamics into the desired signal, making the problem even more difficult. Finally, the problem remains of respiration harmonics falling close to the heartbeat frequency so as to make reliable heart rate estimation difficult. What is needed in the art is a way to overcome these problems. More specifically, what is needed in the art are CP signal processing systems and methods that ameliorate some or all of the above-described disruptive effects and an improved Doppler radar that takes advantage of the systems and methods.